Development of multiscale microbial kinetic-transport models for prediction and optimization of biogenic coalbed methane production
The fundamental objective of this research project is to develop an enzymatic reaction kinetic model for coal bioconversion which, on integration with multiscale transport models, would allow simulation and optimization of field scale biogenic coalbed methane production.
Biogenic coalbed methane (CBM) is an unconventional source of natural gas produced by microbial anaerobic breakdown of coal. Given the many advantages of converting coal to natural gas, much research has been conducted on the enhancement of this natural process at the laboratory scale, and some field scale tests have also been conducted. Commercialization of any such technology requires conceptualization and optimization of field scale strategies. This is a challenge given the complexity and variability of coal, the associated transport processes and the microbial processes involved. In this study, we have used a scaling-up approach starting with the development of reaction kinetics at the smallest scale and the addition of appropriate transport effects at each successive scale to build a model for simulation of CBM in coalbed reservoirs.
The first challenge is to develop a suitable microbial kinetic model with reasonable predictive capability. To this end, microbial reaction networks involved in coal bioconversion were extensively reviewed and complemented with analysis and interpretation of data from laboratory experiments to propose a simplified reaction pathway. An enzymatic reaction kinetic model based on simple and modified Monod models was then derived using lumped species, an approach common in kinetic descriptions of complexreaction mixtures such as those found in fluid catalytic cracking. The model was then validated by nonlinear regression of data from various coal enrichment cultures.
The kinetic model was next applied to a coreflooding experiment, which is a laboratory scale representation of field conditions, with the inclusion of gas diffusion and sorption behaviour. The model was simplified using computational singular perturbation analysis and an optimal model-based experimental design was devised.
Next, a set of partial differential equations were derived to model the multiple gas transport/storage processes occurring in a coalbed reservoir characterized by dual porosity. After discretization using forward difference formulas and non-dimensionalization, the stiff transport model was solved using the Levenberg-Marquardt algorithm. Dimensionless numbers derived in the course also allow analysis of dominant processes at changing scales. History matching of the transport model was performed against gas production data from Manville wells in Alberta. Finally, gas transport and reaction kinetics were coupled for simulation of biogenic coalbed methane flow and then advanced to multiphase, multicomponent reservoir simulations in CMG STARS for estimation and optimization of commercial biogenic coalbed methane production. Polynomial chaos expansion (PCE) was used to quantify the effect of parametric uncertainty in the model on estimates of methane production.